1. Field of the Invention
This invention relates to a toner composition of negative charging property for use in developing an electrostatic latent image in electrophotography. More particularly, the invention is concerned with magenta, yellow, and cyan toners useful in color electrophotography.
2. Description of the Prior Art
U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910, Japanese Patent Publication No. 43-24748, and others describe various methods for electrophotography. In general, the electrophotographic technique is carried out by first forming a latent image on a photosensitive body by various electrical means or expedients using a photoconductive substance, then developing the thus formed electrical latent image by a toner, transferring the developed and powdered image onto a recording material such as paper, depending on necessity, and finally fixing the powdered image by heating or with a solvent vapor, and so forth.
On the other hand, there have been known various methods of obtaining color images by these conventional electrophotographic techniques, wherein an original image is first exposed through a color resolving filter, and an electrostatic latent image thus obtained is developed by the use of a color toner (magenta toner, cyan toner, and yellow toner) corresponding to the color resolving filter, through which the original image has been exposed. The toner images of each color obtained by carrying out these process steps are then superimposed to produce the final color image.
The abovementioned magenta toner, cyan toner, and yellow toner to be used for such color electrophotography, unlike the black toner being used in the ordinary electrophotography, are required to satisfy various conditions as mentioned below.
1. To have high transparency for the sake of superimposing multi-color images.
2. To have preferential or selective electric charge, either positive or negative.
3. The formed image has excellent stability against heat and light, and excellent anti-bleeding property.
4. The toner has excellent spectral reflective characteristics to reproduce an original image with high fidelity.
Unfortunately, however, it is considerably difficult to satisfy these conditions simultaneously. Particular difficulty resides in obtaining a color toner which exhibits a negative electric charging property, has high transparency, and good spectral and reflective characteristics simultaneously.
Heretofore, the negatively charging toner has been produced by mixing metal-containing dyes (reddish black, bluish black) capable of imparting the negatively charging property to the toner (as disclosed in Japanese Patent Publication Nos. 41-20153, 43-27596, 44-6397 and 45-26478) with a binder resin for the toner together with carbon black, etc. While this process holds good with the production of black toners having the negatively charging property, it does not give favorable results in obtaining color toners since inclusion of the metal-containing dyes in the color toner composition inevitably curtails the color tone of the toner and no toner having high color purity can therefore be obtained.